Scrambling Sequences for Wireless Networks

ABSTRACT

An integrated circuit includes logic configured to generate scrambling sequences, each based on a different scrambling seed, for a smart-utility-network data packet communication. A Hamming distance between any two scrambling sequences is half the length of a PSDU of the data packet or greater.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation application of U.S. patentapplication Ser. No. 13/168,180, filed Jun. 24, 2011, which claims thebenefit of and priority to U.S. Provisional Patent Application No.61/358,250, filed on Jun. 24, 2010, both of which are herebyincorporated herein by reference.

BACKGROUND

Wireless personal area networks (“WPANs”) are used to convey informationover relatively short distances. Unlike wireless local area networks(“WLANs”), WPANs need little or no infrastructure, and WPANS allowsmall, power-efficient, and inexpensive solutions to be implemented fora wide range of devices. Smart Utility Networks (“SUNs”) may operateeither over short ranges such as in a mesh network where utility meterinformation is sent from one utility meter to another or over longerranges such as in a star topology where utility meter information issent to a poletop collection point. The terms WPAN and SUN are usedinterchangeably in this document.

In some cases, a scrambled data packet sent over a SUN may not bereceived successfully. The sender may be informed that the data packethas not been successfully received and thus the sender attempts toretransmit the data packet. However, the scrambling sequence may be thecause of the unsuccessful receipt. Thus, if the data packet isretransmitted with the same or a similar scrambling sequence, it ispossible that the receipt will again be unsuccessful.

SUMMARY

Systems for implementing smart utility networks are described herein. Inat least some disclosed embodiments, an integrated circuit includeslogic configured to generate scrambling sequences, each based on adifferent scrambling seed, for a smart-utility-network data packetcommunication. A Hamming distance between any two scrambling sequencesis half the length of a PSDU of the data packet or greater.

In yet other disclosed embodiments, a device includes a processor andtransceiver coupled to the processor. The transceiver includes aphysical layer for smart-utility-network data packet communication. Theprocessor is configured to generate scrambling sequences, based on ascrambling seed, for a data packet transmitted by the transceiver. AHamming distance between any two scrambling sequences is half the lengthof a PSDU of the data packet or greater.

In other disclosed embodiments a machine-readable storage mediumincludes executable instructions that, when executed, cause one or moreprocessors to generate scrambling sequences, each based on a differentscrambling seed, for a smart-utility-network data packet communication.A Hamming distance between any two scrambling sequences is half thelength of a PSDU of the data packet or greater.

These and other features and advantages will be more clearly understoodfrom the following detailed description taken in conjunction with theaccompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, referenceis now made to the accompanying drawings and detailed description,wherein like reference numerals represent like parts:

FIG. 1A-1B illustrate a packet format and packet header format,respectively, in accordance with various embodiments;

FIG. 2 illustrates an integrated circuit configured to produce ascrambling sequence, in accordance with various embodiments; and

FIG. 3 illustrates a particular machine suitable for implementing one ormore embodiments described herein.

NOTATION AND NOMENCLATURE

Certain terms are used throughout the following claims and descriptionto refer to particular components. As one skilled in the art willappreciate, different entities may refer to a component by differentnames. This document does not intend to distinguish between componentsthat differ in name but not function. In the following discussion and inthe claims, the terms “including” and “comprising” are used in anopen-ended fashion, and thus should be interpreted to mean “including,but not limited to.” Also, the term “couple” or “couples” is intended tomean an optical, wireless, indirect electrical, or direct electricalconnection. Thus, if a first device couples to a second device, thatconnection may be through an indirect electrical connection via otherdevices and connections, through a direct optical connection, etc.Additionally, the term “system” refers to a collection of two or morehardware components, and may be used to refer to an electronic device.

DETAILED DESCRIPTION

The following discussion is directed to various embodiments of theinvention. Although one or more of these embodiments may be preferred,the embodiments disclosed should not be interpreted, or otherwise used,as limiting the scope of the disclosure, including the claims, unlessotherwise specified. In addition, one having ordinary skill in the artwill understand that the following description has broad application,and the discussion of any embodiment is meant only to be exemplary ofthat embodiment, and not intended to intimate that the scope of thedisclosure, including the claims, is limited to that embodiment.

In wireless communications, a scrambler is used to reduce theprobability of having long strings of zeroes or ones and to whiten thetransmitted data (i.e., to make the transmitted data appear morerandom.) Scrambled data packets appear unintelligible to a listenerlacking an appropriate descrambling device. The scrambler may generate ascrambling sequence based on a scrambling seed. In some cases, dependingon the scrambler architecture, certain input sequences (i.e., datapackets), when combined with the scrambling sequence, may result in ascrambled data packet containing a long string of 0s or 1s or a bias tohaving more 0s or more 1s, which may increase the likelihood of an errorwhen receiving the data packet. In the case of such a receipt error, thedata packet must be retransmitted. However, if the data packet isretransmitted using the same scrambling sequence, a similar receipterror is likely. In accordance with various embodiments, when a datapacket is retransmitted, a different scrambling sequence (or thescrambling seed used to generate the scrambling sequence) is selected tominimize the likelihood of a receipt error upon retransmission. In someembodiments, choosing a scrambling sequence with a large Hammingdistance (i.e., the number of positions at which the corresponding bitsare different) when compared to the original scrambling sequence reducesthe likelihood of a receipt error upon retransmission.

A WPAN or low-rate WPAN is a simple, low-cost communication network thatallows wireless connectivity in applications with limited power andrelaxed throughput requirements. The main objectives of a WPAN are easeof installation, reliable data transfer, short-range operation, lowcost, reasonable battery life, and a simple but flexible protocol.

Some characteristics of an illustrative WPAN are:

-   -   Over-the-air data rates of 800 kb/s, 600 kb/s, 400 kb/s, 300        kb/s, 250 kb/s, 200 kb/s, 150 kb/s, 100 kb/s, 75 kb/s, 50 kb/s,        40 kb/s, and 20 kb/s    -   Star or peer-to-peer or mesh operation    -   Allocated 16-bit short or 64-bit extended addresses    -   Optional allocation of guaranteed time slots    -   Carrier sense multiple access with collision avoidance channel        access    -   Low power consumption    -   Energy detection    -   Link quality indication    -   16 channels in the 2450 MHz band, 30 channels in the 915 MHz        band, and 3 channels in the 868 MHz band.        These characteristics are not requirements, and each WPAN may        deviate from the characteristics in one or more ways. Two        different device types can participate in a WPAN: a        full-function device (“FFD”) and a reduced-function device        (“RFD”). The FFD can operate in modes such as serving as a        personal area network (“PAN”) coordinator or a device. A FFD can        communicate with RFDs or other FFDs while a RFD can only        communicate with a FFD. More information can be found at IEEE        Std. 802.15.4-2006 available at        http://www.ieee802.org/15/pub/TG4.html, which is hereby        incorporated by reference.

A utility network or smart utility network (“SUN”) is a low-rate (e.g.,40 kbps to 1 Mbps), low-power WPAN that is designed for applicationssuch as utility metering applications to transmit electricity, gas, andwater usage, and other like data from the customer premises to a datacollection point operated by the utility. For example, utility metersare installed for each residence in a residential neighborhood, and theusage data is sent periodically from each utility meter to a datacollection point, which is an element of the WPAN. The data collectionpoint is connected by fiber, copper wire, or wireless connection to acentral office that collects all the usage data for a region. Usage datais sent either directly from each utility meter to the collection pointor from utility meter to utility meter until the collection point isreached in a star or mesh network formation, respectively.

Orthogonal frequency division multiplexing (“OFDM”) is a modulationtechnique that can be used for the physical layer of the SUN. Table 1illustrates various OFDM options. Option 1 may be generated using a 128point inverse fast Fourier transform (“IFFT”), Option 2 may be generatedusing a 64 point IFFT, and Options 3, 4, and 5 may be generated using32, 16, and 8 point IFFTs, respectively. For oversampling, various sizesof IFFTs, such as 256 point, may be used in various embodiments. Inaccordance with various embodiments, the scrambler is used in the OFDMphysical layer.

TABLE 1 OFDM Options Option 1 Option 2 Option 3 Option 4 Option 5 UnitSampling Rate 1333333.33 666666.666 333333.333 166666.666 83333.3333Samp/sec FFT size 128 64 32 16 8 Tone Spacing 10416.66667 10416.6666710416.66667 10416.66667 10416.66667 Hz FFT Duration 96 96 96 96 96microsec Guard Interval 24 24 24 24 24 microsec Symbol Duration 120 120120 120 120 microsec Symbol Rate 8.33333333 8.33333333 8.333333338.33333333 8.33333333 k Sym/sec Pilot-based Modulation Active Tones 10452 26 14 7 # Pilots tones 8 4 2 2 1 # Data Tones 96 48 24 12 6 # DC nulltones 1 1 1 1 1 Approximate 1.09E+06 5.52E+05 2.81E+05 1.56E+05 8.33E+04Hz Signal BW BPSK 1/2 rate 100.00 50.00 25.00 12.50 6.25 kbps coded and4x repetition BPSK 1/2 rate 200.00 100.00 50.00 25.00 12.50 kbps codedand 2x repetition BPSK 1/2 rate 400.00 200.00 100.00 50.00 25.00 kbpscoded BPSK 3/4 rate 600.00 300.00 150.00 75.00 37.50 kbps coded QPSK 1/2rate 800.00 400.00 200.00 100.00 50.00 kbps coded QPSK 3/4 rate 1200.00600.00 300.00 150.00 75.00 kbps coded 16-QAM 1/2 rate 1600.00 800.00400.00 200.00 100.00 kbps coded 16-QAM 3/4 rate 2400.00 1200.00 600.00300.00 150.00 kbps coded Raw rate (BPSK, 800.00 400.00 200.00 100.0050.00 kbps no coding, no repetition) Suggested 1200 600 400 200 100 kHzChannel Spacing

FIGS. 1A and 1B illustrate a packet and packet header (“PHR”),respectively. A synchronization header (“SHR”) comprises a shorttraining field (“STF”) and a long training field (“LTF”). Asillustrated, the STF and LTF are four and two symbols long respectively,but each can be any size in various embodiments. The STF allows a WPANdevice to perform automatic gain control (“AGC”), packet detection,fractional frequency offset estimation, de-assertion of clear channelassessment (“CCA”) based on CCA modes (CCA Mode 1, 2, or 3), and coarsesynchronization. The LTF allows a device to perform finesynchronization, integer frequency offset estimation, and performchannel estimation. The PHR can be any number of data symbols “M.” In atleast one embodiment, the PHR contains:

A Rate field specifying the data rate of the payload frame (5 bits);

One reserved bit after the Rate field;

A Frame Length field specifying the length of the payload (11 bits);

Two reserved bits after the Frame Length field;

A Scrambler field specifying the scrambling seed (2 bits);

One reserved bit after the Scrambler field;

A Header Check Sequence (“HCS”), an 8-bit cyclic redundancy check(“CRC”) for the data fields only; and

Six tail bits, which are all zeros, for Viterbi decoder flushing.

The PHR is encoded at the lowest data rate supported for each bandwidthoption in at least one embodiment. The physical layer convergenceprotocol service data unit (“PSDU”), which can be any number of datasymbols “L” or bits “N,” carries a media access control (“MAC”) sublayerframe, which comprises a MAC header, MAC payload, and MAC CRC in atleast one embodiment. The PSDU also carries convolutional encodertail-bits, which can be six zeros, and pad-bits if necessary to extendthe data.

For OFDM, the STF and LTF fields comprise the preamble. The PSDU, tailbits, and pad bits are scrambled with a (possibly repeated) 511 lengthframe-synchronous scrambling sequence in at least one embodiment. Thetail bits are then reset to all zeros. In some cases, data packetstransmitted in the SUN may be relatively short (e.g., N is less than 80bits). If a data packet is not received properly, the receiver maybroadcast a request (NACK) to retransmit the data packet. However, if asimilar scrambling sequence is used to scramble the PSDU, tail bits, andpad bits of the retransmitted data packet, the likelihood of improperreceipt remains. Thus, in accordance with various embodiments,maximizing the Hamming distance between the scrambling sequencesimproves the likelihood of proper receipt upon retransmission of thedata packet.

FIG. 2 shows a block diagram of an integrated circuit 200 that producesa scrambling sequence in accordance with various embodiments. Theintegrated circuit 200 may be alternately referred to as a “scrambler.”The scrambler 200 comprises delay elements, including a left-most delayelement 202. The delay elements of the scrambler 200 may be initiallyset to a pre-determined non-zero state based the scrambler field of thePHR. For example, the 2-bit scrambler field may specify the followingfour scrambling seeds, as shown below in Table 2.

TABLE 2 Scrambling seeds for PN9 scrambler Scrambler field MSB Scramblerfield LSB Scrambling seed 0 0 0 0 0 0 1 0 1 1 1 1 0 0 0 0 0 1 1 1 0 0 01 1 0 1 1 1 0 1 1 1 1 1 1 0 1 1 1 1 1 0 0The mapping of scrambler field value to scrambling seed may be stored inmemory of a SUN device or hard-coded in the SUN device. One skilled inthe art will appreciate that the scrambling seeds may be mapped to thescrambler field bits in other ways than shown above.

In accordance with various embodiments, a scrambling sequence isproduced by initializing the scrambler 200 with the scrambling seedspecified by the scrambler field. The leftmost value of the scramblingseed is placed in the leftmost delay element 202. The remaining valuesof the scrambling seed are placed in the following eight delay elements.In some embodiments, the scrambler 200 comprises a linear feedback shiftregister, where the label “PN9” represents the scrambling sequenceoutput and the inputs to the XOR are the fourth delay element from theleft 204 and the ninth delay element from the left 209. The output ofthe XOR is fed back into the leftmost delay element 202 and the value ofeach delay element shifts one delay element to the right. Thus, usingthe first scrambling seed as an example to illustrate the shiftingoperation and calculation of the output bits, the first four output bits(i.e., PN9 ₀ through PN9 ₃) are given by the following operations usingthe first scrambling seed, numbering bits from the left:

the fourth bit XOR the ninth bit (i.e., 0 XOR 1),

the third bit XOR the eighth bit (i.e., 0 XOR 1),

the second bit XOR the seventh bit (i.e., 0 XOR 1), and

the first bit XOR the sixth bit (i.e., 0 XOR 0)

This results in the first four bits of the scrambling sequence being 1,1, 1, and 0.

When the scrambler 200 is initialized with the first scrambling seed(i.e., 0 0 0 0 1 0 1 1 1), the first 80 bits of the scrambling sequencegenerated by the scrambler 200 are:

-   -   1 1 1 0 0 1 1 0 0 0 0 1 0 0 1 0 0 0 1 0 1 0 1 1 1 0 1 0 1 1 1 1        0 0 1 0 0 1 0 1 1 1 0 0 1 1 1 0 0 0 0 0 0 1 1 1 0 1 1 1 0 1 0 0        1 1 1 1 0 1 0 1 0 0 1 0 1 0 0 0.

When the scrambler 200 is initialized with the second scrambling seed(i.e., 0 0 0 0 1 1 1 0 0), the first 80 bits of the scrambling sequencegenerated by the scrambler 200 are:

-   -   0 0 1 1 1 0 1 1 1 0 1 0 0 1 1 1 1 0 1 0 1 0 0 1 0 1 0 0 0 0 0 0        1 0 1 0 1 0 1 0 1 1 1 1 1 0 1 0 1 1 0 1 0 0 0 0 0 1 1 0 1 1 1 0        1 1 0 1 1 0 1 0 1 1 0 0 0 0 0 1.

When the scrambler 200 is initialized with the third scrambling seed(i.e., 1 0 1 1 1 0 1 1 1), the first 80 bits of the scrambling sequencegenerated by the scrambler 200 are:

-   -   0 0 1 1 1 1 0 1 0 1 0 0 1 0 1 0 0 0 0 0 0 1 0 1 0 1 0 1 0 1 1 1        1 1 0 1 0 1 1 0 1 0 0 0 0 0 1 1 0 1 1 1 0 1 1 0 1 1 0 1 0 1 1 0        0 0 0 0 1 0 1 1 1 0 1 1 1 1 1 0.

When the scrambler 200 is initialized with the fourth scrambling seed(i.e., 1 0 1 1 1 1 1 0 0), the first 80 bits of the scrambling sequencegenerated by the scrambler 200 are:

-   -   1 1 1 0 0 0 0 0 1 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 0 1 1 1 0 0 0        0 1 0 1 1 0 0 1 1 0 1 1 0 1 1 1 1 0 1 0 0 0 0 1 1 1 0 0 1 1 0 0        0 0 1 0 0 1 0 0 0 1 0 1 0 1 1 1.

Scrambled bits of the data packet are generated by an XOR operation ofeach bit of the PSDU, tail bits, and pad bits (collectively “inputbits”) with the scrambler sequence generated by the scrambler 200 usingthe scrambling seed identified by the scrambler field of the PHR. Inother words, scrambled bit_(n)=input bit_(n) XOR PN9 _(n). In somecases, retransmission of a data packet may be necessary. For example,generating the scrambled bits using a particular scrambler sequence mayresult in poor scrambling properties, such as having long strings of 0sor 1s or a sequence that does not appear to be white or random. Uponretransmission, altering the scrambler sequence used to generate thescrambled bits may reduce the likelihood of unsuccessful receipt sincethe input bits remain the same.

If retransmission is necessary, a different scrambling seed than thescrambling seed used to generate the particular scrambler sequence forthe prior transmission is selected. In some embodiments, the MAC changesthe scrambler seed by setting the scrambler field to “00” when the PHYis initialized and increments the scrambler field, using a 2-bitrollover counter, for each frame sent by the PHY. Thus, the scramblerfield is “00” for the first frame sent, “01” for the second frame, “10”for the third frame, “11” for the fourth frame, and “00” for the fifthframe and so on. In other embodiments, the MAC sets the scrambler fieldto “00” whenever a packet is sent for the first time. When a firstretransmission is requested for a packet because it was not receivedcorrectly, the field is incremented to “01”. The scrambler field is setto “10” If a second retransmission is needed, “11” if a thirdretransmission is needed, and “00” for a fourth retransmission and soon.

As explained above, it is advantageous to maximize the Hamming distancebetween the scrambling sequences so that the data packet is more likelyto be properly received upon retransmission. In a SUN, manyshorter-length data packets are used to communicate control informationand the like, such as acknowledgements (ACKs) or negativeacknowledgements (NACKs). In accordance with various embodiments,diversity-enabled scrambling sequences are optimized to improve thelikelihood that these short packets can be received successfully whenretransmission is requested.

For the 80-bit scrambling sequences listed above, the Hamming distancesfrom the first scrambling seed to the second, third and fourthscrambling seeds are 47, 46, and 45, respectively; the Hamming distancesfrom the second scrambling seed to the third and fourth scrambling seedsare 45 and 46, respectively; and the Hamming distance from the thirdscrambling seed to the fourth scrambling seed is 47. By utilizingscrambling seeds that maximize the Hamming distance between scramblingsequences and, in particular, give a Hamming distance of about half ofthe length of the sequence or higher, short data packets benefit from agreater scrambling diversity if retransmission is necessary.

One skilled in the art will appreciate that other embodiments may, forexample, utilize a different scrambler such as a PN7 scrambler havingseven delay elements, which produces a 127 length repeating sequence.Additionally, other scrambling seeds may be selected that give Hammingdistances between scrambling sequences of less than half of the lengthof the PSDU while still ensuring scrambling diversity if retransmissionof a data packet is necessary.

The system described above may be implemented on a particular machinewith sufficient processing power, memory resources, and networkthroughput capability to handle the necessary workload placed upon it.FIG. 3 illustrates a particular machine 780 suitable for implementingone or more embodiments disclosed herein. The computer system 780includes one or more processors 782 (which may be referred to as acentral processor unit or CPU) that are in communication with amachine-readable medium 787. The machine-readable medium 787 maycomprise memory devices including secondary storage 784, read onlymemory (ROM) 786, and random access memory (RAM) 788. The processor isfurther in communication with input/output (I/O) 790 devices and networkconnectivity devices 792. The processor may be implemented as one ormore CPU chips.

The secondary storage 784 is typically comprised of one or more diskdrives, tape drives, or optical discs and is used for non-volatilestorage of data and as an over-flow data storage device if RAM 788 isnot large enough to hold all working data. Secondary storage 784 may beused to store programs and instructions 789 that are loaded into RAM 788when such programs are selected for execution. The ROM 786 is used tostore instructions 789 and perhaps data, which are read during programexecution. ROM 786 is a non-volatile memory device that typically has asmall memory capacity relative to the larger memory capacity ofsecondary storage. The RAM 788 is used to store volatile data andperhaps to store instructions 789. Access to both ROM 786 and RAM 788 istypically faster than to secondary storage 784.

I/O 790 devices may include printers, video monitors, liquid crystaldisplays (LCDs), touch screen displays, keyboards, keypads, switches,dials, mice, track balls, voice recognizers, card readers, paper tapereaders, or other well-known input devices. The network connectivitydevices 792 may take the form of modems, modem banks, ethernet cards,universal serial bus (USB) interface cards, serial interfaces, tokenring cards, fiber distributed data interface (FDDI) cards, wirelesslocal area network (WLAN) cards, radio transceiver cards such as codedivision multiple access (CDMA) and/or global system for mobilecommunications (GSM) radio transceiver cards, and other well-knownnetwork devices. These network connectivity 792 devices may enable theprocessor 782 to communicate with an Internet or one or more intranets.With such a network connection, the processor 782 may receiveinformation from the network, or may output information to the networkin the course of performing the above-described method steps. Suchinformation, which is often represented as a sequence of instructions789 to be executed using processor 782, may be received from and outputto the network, for example, in the form of a computer data signalembodied in a carrier wave

Such information, which may include data or instructions 789 to beexecuted using processor 782 for example, may be received from andoutput to the network, for example, in the form of a computer databaseband signal or signal embodied in a carrier wave. The basebandsignal or signal embodied in the carrier wave generated by the networkconnectivity 792 devices may propagate in or on the surface ofelectrical conductors, in coaxial cables, in waveguides, in opticalmedia, for example optical fiber, or in the air or free space. Theinformation contained in the baseband signal or signal embedded in thecarrier wave may be ordered according to different sequences, as may bedesirable for either processing or generating the information ortransmitting or receiving the information. The baseband signal or signalembedded in the carrier wave, or other types of signals currently usedor hereafter developed, referred to herein as the transmission medium,may be generated according to several methods well known to one skilledin the art.

The processor 782 executes instructions 789, codes, computer programs,scripts which it accesses from hard disk, floppy disk, optical disc(these various disk based systems may all be considered secondarystorage 784), ROM 786, RAM 788, or the network connectivity devices 792.

In an alternative embodiment, the system may be implemented in anapplication specific integrated circuit (“ASIC”) comprising logicconfigured to perform any action described in this disclosure withcorresponding and appropriate inputs and outputs or a digital signalprocessor (“DSP”) similarly configured. Such logic is implemented in atransmitter, receiver, or transceiver in various embodiments.

The above disclosure is meant to be illustrative of the principles andvarious embodiment of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. Additionally, audio or visualalerts may be triggered upon successful completion of any actiondescribed herein, upon unsuccessful actions described herein, and uponerrors. Also, the order of actions can be varied from order described,and two or more actions may be performed concurrently. It is intendedthat the following claims be interpreted to embrace all variations andmodifications.

What is claimed is:
 1. An integrated circuit, comprising: logicconfigured to generate scrambling sequences, each based on a differentscrambling seed, for a smart-utility-network data packet communication;wherein when a retransmission is requested for a packet, a scramblingsequence different from the last scrambling sequence is used in theretransmission of the packet; and wherein when retransmission isrequested for a packet, a media access control (MAC) changes a scramblerseed by setting a scramble field to “00” when a physical layer (PHY) isinitialized and thereafter increments the scrambler field using a 2-bitrollover counter.
 2. The integrated circuit of claim 1 wherein thelength of a protocol service data unit (PSDU) is 80 bits or less.
 3. Theintegrated circuit of claim 1 wherein the logic comprises a PN9scrambler.
 4. The integrated circuit of claim 3 wherein the scramblingseed is selected from the group consisting of: 000010111, 000011100,101110111, and
 101111100. 5. The integrated circuit of claim 1 whereinthe logic comprises a PN7 scrambler.
 6. The integrated circuit of claim1 wherein the logic is implemented in an orthogonal frequency divisionmultiplexing (OFDM) physical layer.
 7. An apparatus, comprising: atransceiver with a physical layer for smart-utility-network data packetcommunication; and a processor coupled to the transceiver and configuredto generate scrambling sequences, each based on a scrambling seed, for adata packet transmitted by the transceiver; wherein when aretransmission is requested for a packet, a scrambling sequencedifferent from the last scrambling sequence is used in theretransmission of the packet; and wherein when retransmission isrequested for a packet, a media access control (MAC) changes a scramblerseed by setting a scramble field to “00” when a physical layer (PHY) isinitialized and thereafter increments the scrambler field using a 2-bitrollover counter.
 8. The apparatus of claim 7 wherein the length of thePSDU is 80 bits or less.
 9. The apparatus of claim 7 wherein theprocessor generates the scrambling sequences using a PN9 scrambler. 10.The apparatus of claim 9 wherein the scrambling seed is selected fromthe group consisting of: 000010111, 000011100, 101110111, and 101111100.11. The apparatus of claim 7 wherein the processor generates thescrambling sequences using a PN7 scrambler.